Multi-layer circuit devices such as micro-fluid ejection heads have a plurality of electrically conductive layers separated by insulating dielectric layers and applied adjacent to a substrate, typically a semiconductor substrate. Thermal energy generators or heating elements, usually resistors, are located on an ejection head chip and are for heating and vaporizing fluid to be ejected.
Micro-fluid ejection devices such as ink jet printers continue to experience wide acceptance as economical replacements for laser printers. Micro-fluid ejection devices also are finding wide application in other fields such as in the medical, chemical, and mechanical fields. As the capabilities of micro-fluid ejection devices are increased to provide higher ejection rates, the ejection heads, which are the primary components of micro-fluid ejection devices, continue to evolve and become larger, more complex, and more costly to manufacture.
One significant obstacle to be overcome in micro-fluid ejection head manufacturing processes is maintaining the planarity of the ejection device substrate, also referred to as the ejection chip, and the nozzle plate during and after the manufacturing process. The planarity of the ejection chip and the nozzle plate, (hereainafter referred to as “ejection head chip”) determines the direction in which a fluid such as ink is dispensed. If the nozzle plate is warped or bowed, due to warping or bowing of the underlying ejection device substrate, the desired direction of fluid-jetting is compromised. The planarity of these components may be affected by mismatched coefficients of thermal expansion between the various members of the ejection head, including the nozzle plate, the device substrate, the base support, and any adhesive material used in securing the aforementioned components to one another.
Current manufacturing processes are limited by the size of the ejection head substrate used to provide a single ejection head chip. In order to provide higher speed or quantity of fluid ejection, larger ejection heads are needed. Larger ejection heads may be provided by attaching multiple chips to a single substrate. However, mounting multiple chips on a single substrate increases the difficulties of maintaining manufacturing tolerances. For example, the difficulty of maintaining the planarity and manufacturing tolerances of multiple chips on a substrate is greatly increased as the number of chips on a substrate increases.
During the manufacturing process, a polymeric die attach adhesive is typically used to secure the components of the ejection head to one another. However, such adhesives require thermal curing which causes expansion and contraction of the components and may lead to warping or bowing of the ejection device substrate and the nozzle plate. Alterations in the thickness of the adhesive layer or the thickness of the underlying support material have led to only marginal improvements in the planarity of the finished devices.
Ceramic substrates, commonly high purity alumina, have been used for mounting multiple ejection head chips because of their dimensional stability and rigidity. Ceramic substrates are generally formed in a “green”, pliable, unfired state and then fired prior to mounting the chips on the substrate. During firing, shrinkage occurs, leading to poor control over dimensional tolerances in the as-fired state. Accordingly, subsequent lapping may be required to provide a suitably planar surface for mounting the ejection head chips.
Another tolerance parameter for mounting multiple ejection head chips on a single substrate is that the ejection head chips have bond pads on the same surface as the ejectors for connection to wiring typically provided on a flexible circuit or printed circuit board (PCB). Accordingly, it is desirable for the surface surrounding the ejection head chips to be in substantially the same plane as the ejector surface for effective wiping, maintenance, and capping. Therefore chips have often been mounted in recessed “pockets” to facilitate maintenance functions and to allow for interconnection to wiring. Providing a planar die attach surface for mounting multiple chips in recessed pockets is difficult and increases the difficulty of manufacturing large, multi-chip ejection heads. Accordingly, there is a need to improve the manufacturing techniques and tolerances for making multi-chip micro-fluid ejection devices.
In view of the foregoing and other needs, an exemplary embodiment of the disclosure provides a composite ceramic substrate for receiving an ejection head chip or chips for a micro-fluid ejection head. The substrate includes a ceramic base having a substantially planarized first surface and at least one fluid supply slot therethrough. A low temperature co-fired ceramic (LTCC) tape layer bundle having at least two LTCC tape layers is attached to the ceramic base at an interface between the LTCC tape layer bundle and the first surface of the ceramic base. The LTTC tape layer bundle has at least one opening therein providing side walls of a chip pocket when attached to the ceramic base and at least one of the LTCC tape layers includes a plurality of conductors for providing electrical connections to the ejection head chip in the chip pocket.
Another exemplary embodiment of the disclosure provides a method for fabricating a micro-fluid ejection head structure. According to the method, conductors are applied to a surface of at least one low temperature co-fired ceramic (LTCC) tape layer having a chip pocket, opening therein. A bundle of two or more green LTCC tape layers having chip pocket openings therein including the LTCC tape layer having the conductors thereon is formed. The bundle of LTCC tape layers is attached to a substantially planarized surface of a previously fired ceramic base to provide a composite ceramic structure. The composite ceramic structure is then fired at a temperature ranging from about 800° to about 1000° C. to provide the micro-fluid ejection head structure having encapsulated conductors therein.
An advantage of the composite ceramic structure according to the disclosure is that a substantially planar surface of a previously fired ceramic material base may be provided for improved planarity of micro-fluid ejection head chips attached to the base. Additionally, the LTCC layer bundle provides improved encapsulation of conductors after tiring the ceramic base. Use of LTCC layers to provide the LTCC layer bundle also enables the use of relatively low resistance conductor material to provide the encapsulated conductors lines.
By comparison, micro-fluid ejection beads using substrates made of high temperature co-fired (HTCC) tape layers, as described in U.S. Patent Publication Nos. 2002/0033861, 2004/0113996, and U.S. Pat. No. 6,543,880, are fired at temperatures of about 1600° C., and thus require the use of refractory metals that have relatively high resistance. Use of the LTCC layers for encapsulating the conductors enables the use of relatively lower firing temperatures and the use of non-refractory metals for conductors. Another advantage of the LTCC layers is that LTCC materials are available that have a shrinkage rate in the X-Y plane of less than about 1%. Since the LTCC layers may be laminated to a base ceramic substrate at temperatures substantially below 1600° C., dimensional changes and/or warpage of the base ceramic and delamination between the base ceramic and LTCC layers is minimized.